Modern manufacturing processes sometimes require precise stoichiometric ratios of chemical elements during particular manufacturing phases. To achieve these precise ratios, different process gases may be delivered into a process chamber during certain manufacturing phases. A gas panel may be used to deliver these process gasses to a process tool with one or more chambers or reactors. A gas panel is an enclosure containing one or more gas pallets dedicated to deliver process gases to the process tool. The gas panel is in turn composed of a group of gas pallets, which is itself composed of a group of gas sticks.
A gas stick assembly may contain several discrete components such as an inlet fitting, manual isolation valve, binary controlled pneumatic isolation valves, gas filters, pressure regulators, pressure transducers, inline pressure displays, mass flow controllers and an outlet fitting. Each of these components is serially coupled to a common flow path or dedicated channel for one particular process gas. A manifold and a valve matrix channel the outlet of each gas stick to the process chamber.
To achieve a certain stoichiometric ratio, a process tool controller asserts setpoints to the mass flow controllers, and sequences the valve matrices, associated with certain gas sticks. The indicated flow value is output by the mass flow controller of each gas stick and monitored by the process tool controller.
A mass flow controller (MFC) is constructed by interfacing a flow sensor and proportioning control valve to a control system. The flow sensor is coupled to the control system by an analog to digital converter. The control valve is driven by a current controlled solenoid valve drive circuit. A mass flow measurement system is located upstream of the control valve. The control system monitors the setpoint input and flow sensor output while refreshing the control valve input and indicated flow output. Closed loop control algorithms executed by the embedded control system operate to regulate the mass flow of process gas sourced at the inlet fitting through the proportioning control valve and outlet fitting such that the real-time difference or error between the setpoint input and indicated flow output approaches zero or null as fast as possible with minimal overshoot and as small a control time as possible. As over 500 species of gases may be used in the manufacturing of certain electronic components, the operation of each of the respective mass flow controllers is critical. Typically, these mass flow controllers are validated using the process chamber itself. FIG. 1 depicts one such prior art system where process chamber 130 is used as a flow verification tool. To verify mass flow controller 120, a setpoint signal is input to mass flow controller 120 which in turn begins flowing gas to process chamber 130. As the volume of process chamber 130 is known, a primary flow measurement technique known as rate-of-rise may be utilized to measure the flow into that volume. This method utilizes the conservation of mass principle and the equation of state of the gas to derive a relationship between the pressure in a fixed volume and the flow (mass flow) into that volume. The equation is given as,
                              m          .                =                              [                                          Δ                ⁢                                                                  ⁢                                  P                  ·                  V                                            RT                        ]                                Δ            ⁢                                                  ⁢            t                                              eq        .                                  ⁢                  (          1          )                    where ΔP is the change in pressure over the interval Δt, R is the universal gas constant, T is the absolute temperature of the gas, and V is the volume of the measurement chamber. Eq. 1 utilizes the ideal gas equation as the equation of state; similar equations can be derived for other equations of state.
Unfortunately, the volume of typical process chamber 130, which may be on the order of 20 to 60 liters makes measurements of small flow extraordinarily time consuming. Additionally, process chamber 130 may exhibit large temperature gradients throughout its volume, distorting both the measurement and calculation of the mass flow into process chamber 130.
FIG. 2 shows the amount of time required to achieve a given change in pressure for some typical flow rates using typical process chamber 130 of between 20 and 60 liters. Due to many other constraints, a minimum pressure of 0.1 Torr may be required to initiate the measurement, and 0.3 Torr minimum accumulated pressure required to make the measurement. As a result, to perform a single flow point validation of a 2 sccm flow can require up to 5 minutes and verification of a mass flow controller may then take as long as 30 minutes. This lengthy validation cycle decreases the tool availability and adds cost to the user.
In addition to the slowness of the measurement, the accuracy of the measurement is typically no better than +/−5% of the reading. The primary contributing errors are: errors in temperature, errors in chamber volume, and unaccounted for gases (adsorption or desorption).
Other methods of validating mass flow controller 120 may utilize a secondary volume in parallel to process chamber 130 to measure flow. However, these methods do not allow the measuring of transient (non steady-state) performance of mass flow controller 120, and the many steps required to determine the volume upstream of mass flow controller 120 make this technique difficult to integrate into existing systems and may exacerbate already long time requirements for validation.
Thus, there is a need for systems and methods for validating a mass flow controller which can quickly measure dynamic performance and validate a mass flow controller, while simultaneously improving the accuracy of the validation process by reducing measurement uncertainties.